This invention relates to plating of copper-refractory metal substrates, such as copper-tungsten substrates with a nickel/gold plating.
Material made of copper and a refractory metal such as tungsten or molybdenum is used as the carrier substrate for gallium arsenide microelectronic chips. The copper-refractory metal material has a relatively high thermal conductivity, which permits the rapid removal of heat from the chips. It also has a coefficient of thermal expansion that is closely matched to that of the gallium arsenide chip, so that thermal mismatch strains are small.
The substrate is plated with a nickel/gold coating prior to attachment of the microelectronic chips. The nickel/gold coating includes one or more nickel layers adjacent to the surface of the substrate, and a gold layer overlying the nickel layer(s). The gold coating allows the chip to be soldered to the surface of the substrate, and protects the surface of the substrate against oxidation or corrosion damage. The nickel layer(s) improve the adherence of the gold layer to the substrate and act as a diffusion barrier of copper to the gold layer.
In the conventional plating approach, the substrate is prefired at elevated temperature, typically about 850.degree. C., in hydrogen to remove surface metal oxides. It is next chemically activated by dipping into concentrated hydrochloric acid to remove additional metal oxides. The substrate is thereafter plated with a nickel strike layer and furnace sintered in hydrogen at about 850.degree. C. to improve the adherence of the nickel strike layer to the substrate. The strike-plated-and-sintered substrate is again chemically activated by dipping into concentrated hydrochloric acid. A nickel plating is plated over the activated nickel strike layer, and the gold layer is plated over the nickel layer.
The conventional approach is operable, but has drawbacks. Blisters are sometimes observed to form on the plated surface during processing. These blisters are believed to result from the rapid formation of tenacious oxides on the surface of the substrate that prevent the adherence of the overlying layers. The resulting substrate having a blistered nickel/gold coating is not acceptable for use in the carrier application, inasmuch as the coating may spall away, the gallium arsenide chip may not adhere properly to the substrate, and the blistered surface may cause the chip to be cracked.
There is a need for an improved approach to the coating of copper-refractory metal materials, such as those used in microelectronic applications as chip carriers. The present invention fulfills this need, and further provides related advantages.